Epoxy compositions are widely used as coatings, adhesives, structural components and encapsulants in the manufacture of integrated circuit assemblies. As thermosetting compositions, the epoxy materials are difficult or impossible to remove after curing. If a wirebonded chip, encapsulated with epoxy, fails, there is currently no way to remove the encapsulant, remove the chip and replace it with a good chip. The board, therefore, usually has to be discarded.
Representative methods and apparatus for reworking packaging and adhesives include Gutierrez, U.S. Pat. No. 5,371,328 which teaches rework of circuit modules containing unusable electronic components which were previously coated. Gutierrez teaches use of a non-stick release layer applied between the chip and the substrate and inhibits the epoxy from adhering to the substrate. Grebe, U.S. Pat. No. 5,274,913 also teaches a method of fabricating a reworkable module using a release layer under the chip encapsulant, such as, flip chip or ball grid array.
Juskey, et al., U.S. Pat. No. 5,232,758 teaches non-hardening solvent removal hydrophobic conformal coatings using a wax. Generally the encapsulant comprises a metal alkyl benzyl sulfonate, which are dispensed by several common methods over electronic components, to impart a water resistant coating. The wax may be removed by dissolution in any number of non-polar low molecular weight hydrocarbons.
Ohta, et al., U.S. Pat. No. 4,822,441 discloses a plastic mold decapsuling apparatus taught to remove the plastic molding compound from molded components. The tool consists of two pumps, a plunger pump for moving nitric acid to a reservoir and a gear pump for moving it to the mold holder. High pressure solvent flow is used to remove plastic molding over circuit assemblies.
Minetti et al., U.S. Pat. No. 3,969,813 depicts a method and apparatus for removal of semiconductor chips from hybrid circuits. The Minetti tool consists of 4 or 5 jet streams directed at adjacent sides of the encapsulated element which has a top and bottom surface. High pressure jet streams in the range of 12,000 to 20,000 psi, are deflected off the substrate to the bottom surface of the element and tend to lift the element off the surface.
Wensink, U.S. Pat. No. 4,344,809, discloses a jet etch apparatus for decapsulation of molded devices. The Wensink tool is used to remove the epoxy molding compound from an encapsulated electronic component. Ellerson, et al., U.S. Pat. No. 5,252,179 discloses an apparatus and method for selectively etching plastic encapsulating material. Ellerson discloses a tool for removing encapsulant from underneath a chip or ball grid array package which relies upon positioning a shielding fixture over the top surface of the electronic device.
Some of these processes and assemblies use easily removable or dissolvable encapsulants. However, when a cured thermoset encapsulant is used, prior efforts have included assemblies which use high solvent pressure and flow rate. These devices may tend to have a deleterious effect on the circuit assembly and may not allow for secure encapsulation and nondestructive repair.
As a result, there is a need for methods and apparatus which will remove integrated circuit elements in a nondestructive manner allowing reuse of the otherwise operable assembly.